1. Field of the Invention
The present invention relates to silane coatings for metals. More particularly, the present invention provides coatings which include an acyloxy silane, and are particularly useful for preventing corrosion and/or promoting adhesion between a metal substrate and a polymer layer applied to the treated metal substrate. Solutions for applying such coatings, compositions as well as methods of treating metal surfaces, are also provided.
2. Description of the Related Art
Most metals are susceptible to corrosion, including the formation of various types of rust. Such corrosion will significantly affect the quality of such metals, as well as that of the products produced therefrom. Although rust and the like may often be removed, such steps are costly and may further diminish the strength of the metal. In addition, when polymer coatings such as paints, adhesives or rubbers are applied to the metals, corrosion may cause a loss of adhesion between the polymer coating and the metal.
By way of example, metallic coated steel sheet such as galvanized steel is used in many industries, including the automotive, construction and appliance industries. In most cases, the galvanized steel is painted or otherwise coated with a polymer layer to achieve a durable and aesthetically-pleasing product. Galvanized steel, particularly hot-dipped galvanized steel, however, often develops “white rust” during storage and shipment.
White rust (also called “wet-storage stain”) is typically caused by moisture condensation on the surface of galvanized steel which reacts with the zinc coating. On products such as GALVALUME®, the wet-storage stain is black in color (“black rust”). White rust (as well as black rust) is aesthetically unappealing and impairs the ability of the galvanized steel to be painted or otherwise coated with a polymer. Thus, prior to such coating, the surface of the galvanized steel must be pretreated in order to remove the white rust and prevent its reformation beneath the polymer layer. Various methods are currently employed to not only prevent the formation of white rust during shipment and storage, but also to prevent the formation of white rust beneath a polymer coating (e.g., paint).
In order to prevent white rust on hot-dipped galvanized steel during storage and snipping, the surface of the steel is often passivated by forming a thin chromate film on the surface of the steel. While such chromate coatings do provide resistance to the formation of white rust, chromium is highly toxic and environmentally undesirable. It is also known to employ a phosphate conversion coating in conjunction with a chromate rinse in order to improve paint adherence and provide corrosion protection. It is believed that the chromate rinse covers the pores in the phosphate coating, thereby improving the corrosion resistance and adhesion performance. Once again, however, it is highly desirable to eliminate the use of chromate altogether. Unfortunately, however, the phosphate conversion coating is generally not very effective without the chromate rinse.
Recently, various techniques for eliminating the use of chromate have been proposed. These include coating the galvanized steel with an inorganic silicate followed by treating the silicate coating with an organofunctional silane (U.S. Pat. No. 5,108,793).
U.S. Pat. No. 5,292,549 teaches the rinsing of metallic coated steel sheet with a solution containing an organofunctional silane and a crosslinking agent.
U.S. Pat. No. 6,071,566 relates to a method of treating a metal substrate to provide permanent corrosion resistance. The method comprises applying a solution containing one or more vinyl silanes in admixture with one or more multi-silyl-functional silanes to a metal substrate in order to form a coating.
Various other techniques for preventing the formation of white rust on galvanized steel, as well as preventing corrosion on other types of metals, have also been proposed. Many of the proposed techniques described in the prior art are, however, ineffective, or require time-consuming, energy-inefficient, multistep processes. Thus, there is a need for a simple, low-cost technique for preventing corrosion on the surface of metal.
A particular problem associated with the silane treatments of the prior art is the rate of hydrolysis of the silane compounds. Such compounds are generally hydrolysed in water, at a specific pH, prior to application of the solution to the substrate to be treated. The rate of hydrolysis varies between silanes, and the degree of hydrolysis is a priori not known. Generally, it has to be guessed when the solution is ready for application. When the solution has turned cloudy, this indicates that condensation of the silanes has occurred and the effectiveness of the treatment solution is reduced.
A further problem with the prior art techniques is the inherent insolubility in aqueous media of some of the silanes employed in the metal treatments. To overcome this problem it is commonplace to dissolve the silane with the aid of an organic solvent, for example, alcohols. Thus a final treatment solution commonly contains up to 60% alcohol. The use of many volatile organic compounds (VOCs), including solvents, is highly undesirable from an economic, as well as an environmental perspective. Apart from the cost of such organic solvents, including the cost of their disposal and methods of treatment solution preparation, such compounds present a threat to the environment and are a hazzard to the premises and personnel handling the materials.
A further problem is that the silane systems used in treatment solutions have to have their pH maintained in specific ranges by the initial and continuous addition of acids or bases.
It would therefore be desirable to provide an effective treatment method for metal surfaces, especially to prevent corrosion, and/or improve adhesion.
It would also be desirable to provide a treatment solution useful in preventing corrosion, and/or adhesion promotion of metal surfaces, for example, steel, aluminium, aluminium alloys, zinc, zinc alloys, magnesium, magnesium alloys, copper, copper alloys, tin and tin alloys, particularly zinc, zinc alloys, and other metals having a zinc-containing coating thereon.
It would additionally be desirable to provide a metal surface having improved corrosion resistance and/or improved adhesion characteristics.